"Nature Guide Journal"

8 February 2001

As Jonathan Swift remarked,

"So, naturalists observe, a flea

Hath smaller fleas that on him prey;

And these have smaller fleas to bite 'em,

And so proceed ad infinitum."

Viruses could be considered the "smallest flea" in nature, since they infect all known forms of life, animals, plants and fungi—even bacteria.

Are viruses living things? That question is sometimes hotly debated by students of biology. Viruses could be thought of as life stripped down to it's barest element: reproduction. Viruses do not take in nutrients or pass waste; their only structures and processes are those directly related to holding their reproductive material and injecting it into a suitable host.

A virion (one virus particle) is composed of: genetic material (either DNA or RNA); a protein coat around the genetic material; usually an attachment location or mechanism; and sometimes an encompassing envelope.

Viruses' protein coats have varying durability, causing some to become unable to reproduce in fairly short order if exposed to a foreign environment. Other viruses can remain virulent under a wide range of conditions for great lengths of time.

The virion is totally inert until it encounters an appropriate cell wall. Each kind of virus is limited to infecting a small number of hosts; in some cases, infecting only certain strains of a single species.

At that encounter, the virion injects it's genetic material into the host cell. Usually, the genetic material hijacks the cell's processes and directs it to make several dozen copies of the viral genetic material and other components. The components then assemble and burst out of the cell to disperse, usually killing the host cell.

(The illness we feel with a viral infection, such as influenza, is a combination of the virus killing it's host cells and our bodies' immune response revved up to high-gear.)

Sometimes, particularly with viruses that attack bacteria, the genetic material is incorporated into the host cell. In those cases, the infected bacterium continues to live and reproduce, passing on the virus material.

The viral input may also alter the host bacterium in outward ways. For example, the bacteria that cause botulism, diphtheria, and scarlet fever only effect those diseases when infected themselves with certain viruses.

Occasionally the virus' meddling with the genetic material of bacteria moves material between bacteria. This mixing can serve the same function as sexual reproduction does in more complicated organisms, increasing the genetic diversity of the population.

Although a few chemicals can disable viruses, antibiotics have no effect on these ultimate parasites. In fact, treating a viral infection with antibiotics kills beneficial bacteria that aid in digestion, while selectively breeding for stronger detrimental bacteria. Sometimes the "smallest flea" wins.